A power generator, for example, a photovoltaic power generating system that uses power generated by solar panels in a home environment or the like includes a direct-current voltage bus that transmits power generated by the power generator or power stored in a secondary battery to a power system (hereinafter referred to as HVDC bus). The HVDC bus is connected to a PV converter, inverter, bidirectional DC-DC converter, and the like. The PV converter outputs power generated by the photovoltaic power generating device at a predetermined voltage. The bidirectional DC-DC converter is connected to the secondary battery and converts a direct-current voltage delivered between the secondary battery and HVDC bus into a predetermined constant voltage. In such a photovoltaic power generating system, if the HVDC bus voltage varies because of fluctuations in the amount of power generation in the photovoltaic power generating device, the invertor may be unable to output constant power with stability.
Patent Document 1 (identified below) discloses a power supply system in which a plurality of power supply units, such as photovoltaic power generating units, are connected to a direct-current bus (e.g., a HVDC bus). Each of the power supply units autonomously determines the amount of power delivered between it and the direct-current bus on the basis of a direct-current bus voltage. The power supply system described in Patent Document 1 adjusts the amount of power output by the power supply units by changing the slope of the voltage-current characteristics of each of the power supply units and thus suppresses variation in the direct-current bus voltage.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-224009.
However, with the method for suppressing variation in the direct-current bus voltage by changing the slope of the voltage-current characteristics, as described in Patent Document 1, the control can be quite complex and unstable, and the invertor may be unable to output constant power with stability.